Magnesium base alloy



Patented Dec. 2, 1941 MAGNESIUM BASE ALLOY Joseph D. Hanawalt and Charles E. Nelson, Midland, Mich., assignors to The Dow Chemical. Company, Midland, Mich., a corporation of Michigan No Drawing. Application March 9, 1940',

Serial No. 323,114

1 Claim.

pure magnesium which has a corrosion resist-' ance value of not to exceed about 0.2 milligram per square centimeter per day and the commercial ternary alloy cannot be utilized for many purposes for which it is otherwise well suited.

As is the case with many other pure metals, magnesium itself is not a desirable structural metal and industry has resorted to alloys, among the commonest of which is that of magnesium with aluminum and manganese above described. The commercial alloy of magnesium, aluminum, and manganese is comparatively easily corroded in aqueous sodium chloride solution or by brine spray and is therefore unsuited, for example, for

use along the seaboard or on board ship where it might be exposed to the prevailing atmospheric conditions. The comparatively poor cor- A rosion resistance of the commercial magnesiumaluminum-manganese alloy is not due to the presence of the aluminum or of the manganese but it is due to the existence of additional elements in actually small, but still disadvantageously high proportions. Thus, an alloy consisting of very pure magnesium, pure aluminum, and pure manganese and containing no other elements, has a corrosion resistance at least equal to that of magnesium alone. Such a pure alloy, however, is not as workable as the commercial alloy and further, it is improbable that it could be made generally available economically.

It is an object of the present invention to provide a magnesium-aluminum-manganese alloy which will have a corrosion resistance equal to that of the pure alloy and which, in addition to high corrosion resistance, exhibits workability at least comparable with that of the alloy 'hereprises immersing a weighed sample of the material of measured area into a 3 per cent aqueous sodium chloride solution at room temperature for 2 minutes, withdrawing the sample and holding it in the air for 1 minute, repeating this cycle for a protracted period, and computing the loss in weight ofthe sample per square centimeter of surface area per day. In the standard tests herein reported, the corrosion resistance values are expressed in terms of average weight lms per day per unit area over a testing period of 112 days.

.It has now been found that the foregoing and related objects may be attained and that a magnesium-aluminum-manganese alloy can be produced which will have a corrosion resistance, as measured by the alternate immersion method, at least as good as that of ultra pure magnesium and which exhibits the desired physical properties which industry requires by including in the said alloy a very small 'amount of at least one .of the elements iron, nickel, copper, lead and silicon. It has been recognized that the presence of iron in various magnesium alloys is disadvantageous, and that the amount of iron should be kept as low as possible to minimize corrosion. This is not the whole story, however, and we have found that mere elimination of iron or reduction of the iron content to a very low value is not suflicient to produce a non-corrosive alloy when other elements are present. Iron, even in minute traces, acts co-operatively with various other elements to lessen the corrosion resistance of a magnesium alloy. It has been found, therefore, that not only must the amount of iron be minimized, but also that the amount of numerous other elements must be controlled whether these elements are present alone or together with traces of iron. The presence of at least traces of theother elements is distinctly advantageous from some standpoints, provided the amount thereof is below the corrosion tolerance limit, as will be more fully described hereafter.

The invention, then, resides in an alloy characterized by a degree .of corrosion resistance substantially equal to that of pure magnesium consisting of from 1 to 12 per cent of aluminum, from 0.01 to 0.5 per cent of manganese, and containing at least one of the elements iron, nickel, copper, lead and silicon in amount not to exceed 0.002 per cent iron, not to exceed 0.001 per cent-nickel, not to exceed 0.09 per cent of copper, not to exceed 0.7 per cent of lead, not to 55 exceed 1.0 per cent of silicon, except that the the corrosion rates, determined by the alternate immersion method, are expressed in milligrams weight loss per square centimeter per day based on average loss over a period of 112 days.

2 2,264,308 combined weight of iron and copper, when both TABLE 1 are present, is limited by the expression: Efiect of ck e1 %3e+ se 5 Al Mn Nickel the combined weight of iron and lead, when both me are present, isglirmted tire expression. g 5 8? 5 6332 0 w 0 iron 0 ea 0.002 0.7 31.. 3:5? 8%? 8: and, when the amount of iron is greater than 10 131? 81 318 0% 810% 0.0005 per cent, silicon is less than 0.09 per cent; g-ggi gis the balance being magnesium. An alloy com- 315 0118 010011 0148 posed as above is characterized by having a cor- 8%? rosion rate, in the alternate immersion method 12.0 0Z1 0100a 4113 in brine, of not'to exceed about 0.2 milligram M034 per square centimeter per day.

In the fractional expressions given above, It has been found that the presence of these the numerators represent the weight per cent small amounts of the additional elements in an of the various named constituents actually otherwise pure alloy of magnesium with 1 to 12 present in the alloy based on its total weight. p c nt f aluminum and t p t of The denominators are the individual tolerance n an se is advan a u Th f r e mp limits for the representative element ex ressed the elongation v u s f t e all y a improvedin per cent by weight. Thus, the maximum As evidence of this improvement, the following amount of iron which can be present in a mag representative da a a v n n ast samp s nesium-aluminum-manganese alloy of the proprepared in a uniform manner and tested accordportion hereinbefore given, while still producing to S da d pro edu e: a composition comparable in its corrosion resistance to pure magnesium, is 0.002 per cent. The Alloy beingtested Elongation maximum amount of nickel that can be present it. such an alloy with the same limits as to corro- M 1 ion resistance 5 0.001 per cent. Similarly, the- 13 to M M11 (D Y 12-15 maximum amo nts of copper, lead and silicon are fiig fifi 2?, 3 Fe. {23% 0.09 per cent, 0.7 per cent and 1.0 per cent, rejfig jj- "100015 Fe 1647 ure +0.00l5 to l7l9 spectively. When iron and copper. iron and lead, rum- 0005 Cu 12-15 or iron and silicon are both present, it has been fig ggjlfi-g found that they exhibit a synergetic efiect on the "Pure"+0I001 Pb 124s susceptibility of the alloy to corrosion and that jjgggllig-g i a limit must be imposed upon the combination Pure"+0:0005 s1 12-15 of iron and copper, or of iron and lead such that 253333-39 1 gig: the sum of the quotients of the actual amount Pure"+combin of iron dividedby its tolerance limit plus the 15 actual amount of copper or lead divided by their respective tolerance limits does not exceed unity. The herein-claimed improved alloy, in addition This limitation has been expressed mathemato its improved corrosion resistance and increased tically in the foregoing paragraph. Whenever elongation values, is at least as hard and as strong iron exceeds 0.0005 per cent, silicon must be less as the commercial alloy. than 0.09 per cent. We claim:

The following table illustrates the critical na- A magnesium base alloy including from 1 to 12 ture of the tolerance limit for nickel given in 59 per centum of aluminum and from 0.01 to 0.5 per the preceding discussion of corrosion. The elecentum of manganese, and containing nickel" in ments named in the table are the only ones presa po v amount n t exceeding 0-001 pe centum; ent in significant amounts. The amounts of these balance magnesium, said alloy having a corrosion respective elements in the alloy being tested are resistance below 0.2 milligram per square centiexpressed in per cent by weight of the alloy, and meter per day in alternate immersion in 3 per cent aqueous sodium chloride.

JOSEPH D. HANAWALT. CHARLES E. NELSON. 

